This invention relates to the semiconductor wafer plasma processing and particularly to the generation of high density plasma for such processing.
The dominant plasma sources in the semiconductor equipment industry are of the capacitive and inductive type. Their attractiveness lies in their simplicity. In both of these sources, the RF electric field directly excites electron currents in the plasma. Collisions between the electrons and neutrals heat the electrons by randomizing their energies. In capacitive sources, the plasma source also provides the RF bias to the wafer or substrate, which is usually necessary in processing of high aspect ratio features. The drawbacks of such capacitive sources are lack of independent control of plasma density and wafer bias. In addition, the presence of large voltage swings in the plasma results in a population of energetic electrons that can lead to device damage. Inductive sources, on the other hand, offer the additional degree of freedom of decoupling the plasma source from the wafer bias. In addition, when the RF coil is physically removed from the plasma, these sources allow for production of higher plasma densities than do the capacitive sources. The one major drawback of these inductive sources is their reduced efficiency at high RF power fluxes. The increased plasma density reflects much of the RF energy back to the generator. The issue of electron damage is not removed either. But the damage can be controlled and reduced with careful design and a uniform plasma in contact with the wafer.
Plasma sources employing stationary magnetic fields have also been developed, but have not gained wide-spread acceptance. These include the electron-cyclotron source and the helicon source. Both of these rely on mode-conversion, which is a phenomenon in which an electro-magnetic wave launched from the outside of a vacuum chamber, either micro-wave or RF, excites an electro-static wave or standing wave at a resonance location in the plasma region within the chamber. The electro-static wave heats the electrons. The resonance location for ECR sources is the location where a particular wave frequency and magnetic field strength relationship is satisfied. For helicon sources, satisfying the resonance condition is more complex.
Both ECR sources and helicon sources have lacked acceptance. This can be attributed to the inherent complexity due to the need to generate large magnetic fields throughout the plasma generation space. The remnants of such fields can lead to plasma non-uniformity and damage to the wafer. Such a magnetic field at the wafer can be removed, but at the cost of additional source complexity. Helicon sources are capable of producing extremely high plasma densities. However, because of physical processes that have not been well understood, they exhibit complex behavior such as abrupt density jumps versus RF power, and operation over regions of magnetic field strength and RF power where the source operation is unstable and intermittent. Such behavior is believed to arise from the effects of boundary conditions on the propagation of the waves in the plasma.
More recently, surface wave plasma sources have been introduced. These vary in design, but their salient feature is the existence of a standing electro-magnetic wave on the boundary of the plasma with the dielectric. At a high enough plasma density, this standing electro-magnetic wave mode converts to the electron plasma wave that heats the electrons. With surface wave plasma sources, the mode-conversion location depends on the plasma density and RF frequency. As plasma density increases, the mode-conversion layer moves to the plasma edge, reducing the effectiveness of plasma generation. In this respect, surface wave sources are similar to inductively coupled plasma sources. The maximum achievable plasma density varies linearly with the RF frequency. In order to achieve useful plasma densities, surface wave plasma sources have RF frequencies starting in the range of 800 MHz and higher.
The need for a plasma energy source that efficiently couples energy into a high density plasma remains, particularly where low intensity magnetic field at a substrate is desired.
A primary objective of the present invention is to provide a plasma energy source that efficiently couples energy into a high density plasma, and, more particularly, which does so while maintaining at a low intensity the magnetic field at a substrate being processed by the plasma.
According to the principles of the present invention, a series of RF plasma sources is provided in which the RF electric field is perpendicular to the DC magnetic field and both are in the plane of the dielectric window through which the energy is coupled into the plasma. The energy deposition mechanism is via excitation of the electron cyclotron wave in the plasma. The configuration is such that excitation can be either directly at the plasma-dielectric interface or indirectly by mode conversion of the electromagnetic wave at some distance from the dielectric window. However, the parameters are maintained to exploit the postulated mechanism of mode conversion, and to do so combined with plasma flows in unstable magnetic field configurations, wherein the curvature of the magnetic field lines is such that plasma instabilities will be excited. These generate plasma flows that expel the plasma away from the dielectric window.
The present invention provides all of the advantage of the Helicon and ECR sources in utilizing mode conversion of the electro-magnetic wave to the electro-static wave in the presence of a magnetic field, while providing the ability to do so with reduced magnetic field at the wafer. Further, the invention can be implemented to avoid reduced efficiency at high RF power fluxes. The magnetic field is generated using relatively inexpensive permanent magnets. And unlike the helicon source, the invention does not produce density jumps as the RF power and magnetic field are varied.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description.